1. Field of the Invention
The present invention relates to an optical scanning device which causes a laser beam to be deflected in a predetermined direction and scan a surface of a photosensitive body, and tan image forming apparatus using such an optical scanning device.
2. Description of the Related Art
In an optical scanning device that is used in laser printers and copiers, conventionally known is technology of causing a laser beam emitted from a light source to perform deflection and scanning based on the rotation of a polygon mirror, and determining the write start timing of writing on a photosensitive body based on a detection signal that is output from a BD (Beam Detect) sensor when a part of the laser beam is reflected by a folding mirror via an imaging lens and enters a light-receiving surface of the BD sensor.
For instance, with the conventional technology, attempts have been made for shortening the distance from the polygon mirror to the BD sensor and miniaturizing the device by disposing a cylindrical lens, which has power in the main scanning direction, immediately before the BD sensor.
Nevertheless, with the conventional technology, since the cylindrical lens is given the power to condense the laser beam in the main scanning direction, the scanning speed of the laser beam that enters the BD sensor becomes slow in comparison to the case of not going through the cylindrical lens, and the speed that the electric charge corresponding to the amount of light of the laser beam being accumulated in the BD sensor will also become slow.
Under circumstances where the rise of the BD sensor is slow as described above, if the amount of light of the laser beam changes due to the contamination of the reflective surface of the polygon mirror or the change in the amount of light caused by the laser beam source, the output timing of the detection signal that is output when an electric charge corresponding to the amount of light of the laser beam is accumulated in a predetermined amount in the BD sensor will also change, and there is a problem in that the write start timing of writing on the photosensitive body will fluctuate considerably.
The present invention was devised in view of the foregoing circumstances, and an object of this invention is to provide an optical scanning device and an image forming apparatus which are resistance to changes in the amount of light of the laser beam, and which can accurately obtain a synchronization signal for adjusting the write start timing of writing on the photosensitive body.